C++: Why does bool arithmetic make sense?

Question

For example, what on earth does true+1 means? If it doesn't make sense, why doesn't my G++ raise an error even in -Wall mode?

Answer 1

From the standard:

According to 4.7 (Integral conversions),

4 If the destination type is bool, If the source type is bool, the value false is converted to zero and the value true is converted to one .

In 4.12,

An rvalue of arithmetic, enumeration, pointer, or pointer to member type can be converted to an rvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

So true + 1 means 1 + 1 and false + 1 means 0 + 1 .

Answer 2

true+1 is an integer arithmetic operation. true is converted to 1 and you get 2 .

Answer 3

布尔值在C ++中隐式转换为int

Answer 4

In the declaration

bool x = true + 1;

true is first promoted to int ; then the addition produces 2; finally the 2 is converted to true by the rule that any basic type value X converts to (X != 0) .

The above also covers update expressions such as x += 1 when x is of type bool , because += is defined in terms of = and + .

However, the use of postfix or prefix ++ on bool is deprecated. And the use of postfix of prefix -- is invalid. Quoting the ¹ Holy Standard, “the operand shall not be of type bool .”.

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^{1 ) C++11 §5.3.2/2 in [expr.pre.incr] and §5.2.6/2 in [expr.post.incr].}